D° metal-alkyl complex

W. Scherer and G.S. McGrady (2004) Angewandte Chemie International Edition, vol. 43, p. 1782 - Agostic interactions in d metal alkyl complexes . 

An example of an alkyne insertion involving an early metal complex is shown in Equation 9.72. The insertion of dimethylacetylene into the permethylscandocene-alkyl complex occurs in a manner similar to that for the insertion of olefins into d metal-alkyl complexes. This reaction gave the product of a cis addition. The competition experiment shows that there is no measurable isotope effect of the a-hydrogen, implying that the modified Green-Rooney mechanism is not followed in this case. 

It is possible to let H2 or H+ reaet with a d metal-alkyl complex in a reaction that seems to be an oxidative addition of H2 followed by a reductive elimination of alkane. 

We have seen that the main reason for the instability of metal-alkyl complexes is elimination of an a-, (3- or y-H atom. The two other properties of d" metal-alkyl complexes (n 0) are ... 

Hydrogenolysis of d metal-alkyl complexes by a-bond metathesis... 

Metal-alkyl complexes are characterized in NMR by upfield signals compared to alkanes. Metal-alkyl complexes can undergo a-, 3- or y-elimination, CO, NO, O2, SO2, olefin or alkyne insertion, and react with electrophiles d° metal-alkyl complexes undergo a bond metathesis... 

The activation of C-H bonds by d° metal centers can be investigated very effectively with complexes of the type t-Bu3SiNH)3ZrR, where R is an alkyl or aryl group, as these compounds undergo reversible elimination of hydrocarbon. In these examples, concerted mechanisms involving R-H bond interaction at d° metal imido complexes, either isolable or transient, give metathesis products via R-H elimination. ... 

Bambirra, S., van Leusen, D., Tazelaar, C.GJ. et al. (2007) Rare earth metal alkyl complexes with methyl-substituted triazacyclononane-amide ligands ligand variation and ethylene polymerization catalysis. Organometallics, 26, 1014. 

Liu, B., Ctii, D.M., Ma, 1. et al. (2007) Synthesis and reactivity of rare earth metal alkyl complexes stabilized by aniUdo phosphinimine and amino phosphine ligands. Chemistry —A European Journal, 13, 834. 

Transition-metal-silyl complexes are also formed by the reactions of metal-alkyl complexes with silanes to form free alkane and a metal-silyl complex. Two examples are shown in Equations 4.114 and 4.115. ° The synthesis of silyl complexes by this method has been accomplished with both early and late transition metal complexes. The formation of metal-silyl complexes from late-metal-alkyl complexes resembles the hydrogenolysis of metal-alkyl complexes to form metal hydrides and an alkane. The mechanisms of these reactions are discussed in Chapter 6. In brief, these reactions with late transition metal complexes to form silyl complexes typically occur by a sequence of oxidative addition of the silane, followed by reductive elimination of alkane. An example of this is shown in the coupling of 1,2-bis-dimethylsilyl benzene with a dimethyl platinum(II) complex (Equation 4.114). Similar reactions occur with d° early metal complexes by a a-bond metathesis process that avoids these redox events. For example, the reaction of Cp ScPh with MesSiH, has been shown to proceed through this pathway (Equation 4.115). ... 

In many other cases, oxidative additions of alkanes occur readily to transition-metal-alkyl complexes to generate hydride dialkyl intermediates that subsequently eliminate alkane and form a new metal-alkyl complex. For example, cations related to the alkyl hydrides of iridium formed by oxidative addition undergo reaction with alkanes at or below room temperature to generate new alkyl complexes (Equation 6.34). Cationic platinum complexes undergo similar reactions with substrates containing aromatic and aliphatic C-H bonds (Equation 6.35). " The C-H activation of the platinum complexes has been studied, in part, to understand and to develop systems related to the ones reported by Shilov that lead to H/D exchange, and oxidation and halogenation of alkanes. 

The reaction of a late-transition-metal-alkyl complex containing d-electrons with hydrogen or a hydrocarbon to extrude alkane or arene and generate a new transition-metal-alkyl, -aryl, or -hydride complex usually occurs by a sequence of oxidative addition and reductive elimination. However, this overall transformation could also occur by a CT-bond metathesis-(Sclieme 6.5). It is difficult to determine by experiment if the reaction of a- late-transition-metal-alkyl complex occurs by the sequence of oxidative addition and reductive elimination or by o-bond metathesis. Therefore, theoretical studies have addressed this issue. In most cases, even when the process would involve a relatively high oxidation state, the process has been calculated to occur by oxidative addition and reductive elimination. ... 

Cleavage of the C-C bond to the 3-carbon of late-metal-alkyl complexes is slower and less common than that of d early metal complexes. However, a few examples of 3-aU yl elimination from late metal complexes are known. The reversible intramolecular insertion of an olefin into the platinum-alkyl complex described in Chapter 9 involves an early example of 3-alkyl elimination from a late-metal-alkyl complex. In addition, mild 3-aIkyl elimination has been reported to occur from a ruthenacylobutane complex. In this case the product is a stable alkyl allyl species. - ... 

In some cases, reactions of electrophiles with metal-alkyl complexes possessing d-elec-trons appear to occur by mechanisms. - " In this process, the electrophile can attack the frontside or backside of the alkyl group. Although retention of stereochemistry at the metal-bound carbon from frontside attack at the metal-carbon bond is often observed, inversion of stereochemistry from backside attack at this carbon has also been observed. 

QV. Scherer, G. S. McGrady. Agostic interactions in d(0) metal alkyl complexes. Angew. Chem. Int Ed., 43(14) (2004) 1782-1806. 

Diaconescu PL. Reactions of aromatic N-heterocycles with d°f-metal alkyl complexes supported by chelating diamide hgands. Acc Chem Res. 2010 43 1352-1363. 

Silicon-transition metal chemistry is a relatively new area. The work of Hein and his associates (1941) on Sn—Co derivatives established the possibility of forming bonds between a Group IVB metal and a transition element 139), but it was another fifteen years before CpFe(CO)2SiMej 203), the first of many silyl derivatives, was synthesized. The interest in these compounds derives from (1) comparison with the corresponding alkyl- and Ge-, Sn-, and Pb- transition metal (M) complexes, including the role of ir-back-bonding from filled d orbitals of M into empty d orbitals on Si (or other Group IVB metal), and (2) expectation of useful catalytic properties from such heteronuclear derivatives. 

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